The present invention relates to a method and apparatus for fastening sheet material by self-piercing riveting or clinching. The term xe2x80x9cclinchingxe2x80x9d is also known as xe2x80x9cpress joiningxe2x80x9d or xe2x80x9cintegral fasteningxe2x80x9d.
Methods and apparatus for riveting of the kind in which a self-piercing rivet is inserted into sheet material without full penetration, such that the deformed end of the rivet remains encapsulated by an upset annulus of the sheet material are known.
FIG. 1 is a diagrammatic section of an example of a riveted joint made by such a riveting method in accordance with the invention. A rivet 1 has a head 2 and a shank 3 terminating in an annular edge 4. The shank 3 is initially cylindrical but is flared outwardly into the illustrated shape as the rivet is driven into two overlapping sheets 5,6 located on a suitably shaped die. As shown, the shank and the edge of the rivet 1 remain embedded in the sheet material 5,6 after the rivet has been set.
An improved self-piercing riveting method is described in our European Patent No. 0675774. In this method the sheet material is clamped with substantial force during the riveting operation between a nose of the riveting machine and the die in the region around the rivet insertion location so that there is minimal distortion of the sheet material during the riveting operation. This method has been proved to increase the strength of the riveted joint and reduce the depth of the annular valley 7. However, the relatively high level of clamping force required to achieve the improved joint characteristics means that a significant pressure of hydraulic fluid or a heavy-duty spring is required to apply the force. Furthermore, if reaction forces within the joint resulting from rivet insertion exceed the clamping force, the nose will be pushed back up away from the die. This results in a reduction of the potential residual compressive stress that could be imparted to the region around the rivet.
Joining two sheets of metal by clinching is known, whereby two sheets of metal are deformed into locking engagement using a punch-and-die combination.
An improved clinching method is described in our European Patent No. 0614405. In this method a hollow rivet or tubular slug is inserted into a clinched joint between sheets and the inner end of a shank of the rivet is outwardly deformed within the clinched joint in such a way that it does not penetrate the panels.
In both the above-described methods a C-frame is used to support the riveting apparatus and die. A lower limb of the C-frame supports the die and, in use, deflects a certain distance during the riveting operation as a result of the rivet insertion and clamping forces. This means that in hydraulic clamping systems top-up hydraulic fluid is generally required to maintain the required level of clamping. The slow response of hydraulic fluid systems to the demand for extra loading leads to relatively long cycle times.
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages and to provide for an improved method and apparatus for fastening sheet material by self-piercing riveting or clinching.
According to a first aspect of the present invention there is provided a method for inserting a fastener into sheet material comprising inserting the fastener into at least one sheet without full penetration such that a deformed end of the fastener remains encapsulated by an upset annulus of the sheet material, the sheet material being disposed between a nose and a die of fastening apparatus and the fastener being inserted into the sheet material by means of a punch that is reciprocal relative to the nose, characterised in that, after the fastener is inserted the punch is retracted and a clamping force is applied to the sheet material between the nose and die in the region around the fastener insertion location so as to reduce deformation of the sheet out of its plane in the region around the fastener insertion location.
The clamping of the sheet after insertion of the fastener in this way ensures that favourable compressive stresses are built into the region around the fastener insertion location and, in the case where one or more sheets are being joined, also ensures that fatigue performance of the joints significantly improves in, comparison to joints produced by conventional fastening methods. Retraction of the punch ensures that clamping of the sheet after rivet insertion is applied only by the nose.
The nose may be supported by a support member, movement of the nose relative to the support member in a direction away from the die being prevented during the fastening operation.
The position of the nose in relation to the die is controlled such that very little or no local thickening or swelling of the sheets in the region around the rivet insertion location is permitted. The method means that the sheet material displaced during the rivet insertion operation is constrained within a substantially fixed distance between the nose and the die. This constraint results in increased residual compressive stress being induced into the joint area as a result of the rivet insertion, thereby providing improved fatigue life of the riveted joint.
The distance between the nose and the die is preferably controlled by restraining the nose of the riveting machine so that it is prevented from being pushed back away from the die during the riveting operation, regardless of the force applied. However in the event of deflection of a support frame (such as a C-frame) owing to rivet setting loads, the nose is still able to move so as to follow the die.
A clamping force may also be applied to the sheet material prior to or during insertion of the fastener.
Preferably the clamping force and the force required to insert the fastener are derived from the same actuator.
The clamping force applied after insertion of the fastener may be varied by varying the force applied to insert the fastener.
According to a second aspect of the present invention there is provided a apparatus for by inserting a fastener into sheet material without full penetration such that a deformed end of the fastener remains encapsulated by an upset annulus of the sheet material, said apparatus comprising a nose in which is disposed a reciprocal punch, means for feeding fasteners successively to the nose for insertion by the punch into the sheet material, a die aligned with the punch for deforming the fastener inserted, the sheet material being disposed between the nose and die during the fastening operation, the nose being supported by a support member, characterised in that there is provided means for retracting the punch into the nose after insertion of the fastener so that the punch does not project therefrom and means for applying a clamping force to the sheet material after insertion of the fastener, the force being applied between the nose and die in the region around the fastener insertion location so as to reduce deformation of the sheets out of their planes in the region around the fastener insertion location.
In one embodiment, when the punch has travelled its full extent and the rivet is inserted, a substantial reaction force will have been established in a supporting frame such as, for example, a C-frame. This force is an equal and opposite reaction to the force applied by the punch. With the nose position controlled, retracting the punch away, from the formed joint will result in the frame reaction load being transferred from the punch to the nose. This transfer of load results in the sheet material surrounding the rivet being squeezed between the nose and die with the same substantial force initially used to insert the rivet. Such post rivet-insertion clamping provides for increased residual compressive stresses being imparted into the joint utilising forces already available as a result of rivet insertion and hence removes the need for separate or additional loading. Further, post rivet-insertion clamping allows for the use of clamping forces ranging from zero to moderate during rivet insertion yet still achieves all of the advantages of a joint clamped with substantial force prior to rivet insertion and has been shown to exceed the fatigue results produced by using moderate or high clamping forces.
The restraint device is preferably connected to the nose so as to prevent movement of the nose away from the die during the riveting operation. It may take the form of a mechanical linkage connected between the nose and a fixed surface, the linkage being acted upon by an actuator so that its movement, and therefore movement of the nose away from the die is prevented during the riveting operation. However the device still allows the nose to follow die deflection.
Alternatively the restraint device may be a rotary nut threadedly engaged with a threaded surface of the nose, the nut being rotatable to enable movement of the nose towards and away from the die, rotation in one direction being prevented during the riveting operation so as to prevent movement of the nose away from the die. However, rotation in the other direction, allowing the nose to follow the die, is permitted.
In an alternative preferred embodiment the restraint device is a wedge member disposed between the nose and supporting frame, the wedge member being moveable between an operative position in which it prevents movement of the nose away from the die operation (but still allowing the nose to follow the die if required) and a released position in which such movement is permitted enabling the mechanism to be retracted at the conclusion of the cycle.
In a further alternative preferred embodiment the restraint device is provided by a supply of hydraulic fluid through an inlet connector to the riveting device, the hydraulic fluid applying pressure to control the movement of the nose towards the die, the inlet connector being fitted with a check valve or similar type device that prevents release of fluid pressure and therefore movement of the nose away from the die during the riveting operation.
According to a third aspect of the present invention there is provided a panel clinching method wherein two or more sheets of material are deformed into locking engagement, the sheet material being disposed between a nose and a die of fastening apparatus, the sheet material being deformed by means of a punch that is reciprocal relative to the nose, characterised in that, after the sheets are deformed by the punch, the punch is retracted and a clamping force is applied to the sheet material between the nose and die in the region around the punch insertion location so as to reduce deformation of the sheet out of its plane in the region around the punch in-sertion location.